For #1429, respect unset initial value on JsonObjectAttribute

Src/Newtonsoft.Json/JsonObjectAttribute.cs

@@ -34,11 +34,11 @@ namespace Newtonsoft.Json
     public sealed class JsonObjectAttribute : JsonContainerAttribute
     {
         private MemberSerialization _memberSerialization = MemberSerialization.OptOut;
-        private NullValueHandling _nullValueHandling = NullValueHandling.Include;
-
+        
         // yuck. can't set nullable properties on an attribute in C#
         // have to use this approach to get an unset default state
         internal Required? _itemRequired;
+        internal NullValueHandling? _nullValueHandling;
 
         /// <summary>
         /// Gets or sets the member serialization.
@@ -56,7 +56,7 @@ namespace Newtonsoft.Json
         /// <value>The null value handling.</value>
         public NullValueHandling NullValueHandling
         {
-            get => _nullValueHandling;
+            get => _nullValueHandling ?? default(NullValueHandling);
             set => _nullValueHandling = value;
         }
 

Src/Newtonsoft.Json/Serialization/DefaultContractResolver.cs

@@ -1511,7 +1511,7 @@ namespace Newtonsoft.Json.Serialization
 
             if (property.NullValueHandling == null && containerAttribute is JsonObjectAttribute objectAttribute)
             {
-                property.NullValueHandling = objectAttribute.NullValueHandling;
+                property.NullValueHandling = objectAttribute._nullValueHandling;
             }
 
             if (requiredAttribute != null)